Previous work with radioactivity for diagnosis and therapy of human tumors was focused on the use of by-product sources of radioactivity such as Iodine-131, Y-90 and the like. These are reactor-produced radionuclides.
Tumor specific monoclonal antibodies have been used extensively in nuclear medicine to carry radioactivity to tumors for what will ultimately be a therapeutic and diagnostic purpose. (Larson, Radiology 165:297-304, (1987)). Therapeutic responses have been reported in melanoma (Larson, Radiology 155:487-492, (1985)); lymphomas (Early et al, JNM 28:692 (1987)); Zimmer et al: JNM 28:603, (1987)); hepatomas (Ettinger et al, Cancer Treat Rep 66:289-297, (1982)), using intravenously injected radiolabeled anti-tumor antibodies as the sole modality of therapy. Intracavitary therapy has been applied successfully in ovarian cancer (intraperitoneal injection), as well as pericardium and pleural spaces. (Epenetos, Lancet pp. 1441-1443, (1984)).
At present, the vast majority of studies are being performed using both diagnostic and therapeutic by-product radioactive material. This involves principally beta decay, or internal conversion radionuclidic decay. Iodine-131 is the most common antibody radiolabel, but Indium-111, Ga-67, Tc-99m, Cu-64, and Y-90 have also been widely used. In addition, promising work has been proposed with the alpha-emitters, Bi-212 and Pb212. (Kozak et al, Proc Natl Sci 83:474-476, (1986)).
Recently, investigators have begun to explore positron labeled antibodies because of the superior imaging properties of PET, and the possibility that such radiopharmaceuticals may become useful diagnostic reagents. Recent reports have involved the use of Ga-68 (68 minutes) and F-18, (110 minute half-life) (Otsuka et al; JNM 28:282, 1987) I-124 (4.2 day half-life) (Miraldi, JNM 28:1078, (1987)); and Zr-89 (78 hour half-life) (Eary et al, JNM 27:983, (1986)).
Positron Emission Tomograph offers quantitative abilities that are unique in nuclear medicine in that the concentration of radioactivity can be determined in depth of tissue without interference from radioactivity in surrounding or overlying tissue. This quantitative tomographic imaging is based on the unique physics of positron decay. Also, positron-emitting radionuclides of C-11, 0-15, and F-18, can be readily incorporated into a variety of biomolecules that are excellent in vivo tracers as can be seen in Table 1.
TABLE 1 ______________________________________ Positron Emitting Radionuclides Commonly Used as Radiotracers in Biology Radionuclide Half-life Example of Use ______________________________________ O-15 124 s Blood flow N-13 10. min. Amino acid tracer C-11 20.3 min. dopamine analogue F-18 110 min. glucose analogue ______________________________________
Quantitative imaging of several biologically relevant radiotracers has lead to methods for estimating important biochemical processes non-invasively in vivo: including regional cerebral glucose metabolism (Reivich et al: Circ Res 44:127-137, 1979); blood flow, oxygen extraction and oxygen utilization; (Frackowiak et al JCAT 4:1448-1452, 1980); and dopamine receptor concentration, (Wong et al: Science 234:1558-1563, 1986; Wong et al Science 226:1393-1386, 1984), as examples. The radionuclides of relevance to antibody labeling are of various half-lives, and a partial listing is seen in Table 2 below.
TABLE 2 ______________________________________ Positron Emitting Radionuclides of Relevance to Radiolabelling of Monoclonal Antibodies. (Data from Dillman, MIRD Pamphlet 10, Society of Nuclear Medicine Publishers, New York, 1975) Equilibrium Radio- Positron Decay Absorbed nuclide % Energy* Half-life Dose Constant** ______________________________________ Ga-68 89 .8340 1.13 hours 1.5742 F-18 97 .3942 1.83 hours .5157 Se-73 65 .5664 7.20 hours .8406 Ga-66 56 1.8989 9.3 hours 2.389 Cu-64 19 .2794 12.8 hours .2799 Co-55 81 1.5 18.2 hours -- As-72 17 2.5 26 hours -- Zr-89 22 .90 78.4 hours -- I-124# 25 .9818 101. hours .4578 As-74 29 .5664 430 hours .5695 ______________________________________ % includes all positrons, even when multiple, per decay *energy of most abundant positron **gmrad/microcurie-hour. summed for all particulate energy including low energy xray less than 10 kev. (nonpenetrating radiation) # Production example: 124Te(p, n)I124, 13 Mev protons, requiring 170 mg Te124 target; yield = 176 microcurie/microAmp-hour Production example: 66Zn(p, n)Ga66, estimated production of approximately 200 microcuries/microAmphour, 66ZnO as the target material.
Because these biologically relevant positron emitting radionuclides are short-lived a cyclotron for the production of the radionuclides must be near-by to the clinics where the radioisotopic preparation is injected in vivo. The imaging devices presently in use are highly developmental in nature and complex. There is a need for specialized personnel to operate and maintain the cyclotron, PET scanners and associated computer equipment necessary for image interpretation.
The elegant and unique information obtained with these methods, has led to the proliferation of PET/Cyclotron centers with both radionuclide production and quantitative imaging capability. At present there are 26 centers in the U.S., and 60 in hospitals worldwide. By 1995, it has been estimated that 300 PET/Cyclotron facilities will be in place, most likely related to the unique research opportunities which this machine offers.
In addition to cyclotrons as a source of positron emitters, there are several radiopharmaceuticals that can be produced from a "generator" system, of suitable quality to provide easily accessible radionuclide on a continual basis, in the same way that the ubiquitous Tc-99m is produced on-site for standard nuclear medicine procedures by eluting a column that contains the radioactive parent isotope, MO-99. A list of positron emitters available from generator systems is shown in Table 3 below.
TABLE 3 ______________________________________ Positron Emitting Radionuclides Available from Generator Systems (CRC Press Radiotracers for Medical Applications II: Columbetti Chapter 4 p. 133-168) Daughter Parent Separation System (half-life) (half-life) Column Eluant ______________________________________ Ga-68 (1.12 h) Ga-68 (280 d) Alumina .005M EDTA Rb-32 (1.3 min) Sr-82 (25 d) BioRex70 .3M Acetate Sc-44 (3.9 h) Ti-44 (46 y) Doxex 1 .times. 8 .2M HCl I-122 (3.6 m) Xe-122 (20 h) -- -- As-72 (26 h) Se-72 (8.4 d) -- -- ______________________________________
The periodic table is divided roughly in half into those elements which are neutron rich, and those which are neutron poor. Neutron rich elements (produced in a reactor) decay by competition between beta-minus decay and internal conversion, where-as neutron poor elements (cyclotron produced), decay by positron emission, and in some cases electron capture.
For reasons that are mainly historical, principally related to the greater weapons and power applications of reactors, medical applications of radioisotopes which are produced as a part of this procedure (by-product material) are much more wide spread. This includes therapy applications, in which radionuclides such as I-131, Au-198, P-32 and Y-90 have been used.
In general reactors are placed in remote areas, away from population centers, because the by-product material which is produced as a natural part of the operation of these machines, contains large quantities of long-lived radioactive elements that have the potential for contaminating the environment. Thus medically useful radionuclides that are useful for nuclear medicine applications must have relatively long half-lives or be available in a generator form that can be shipped long distances.
Instead, cyclotrons (accelerators), have been used principally for "atom-smashing" experiments, and have had a major role in the current understanding of the state of matter, and basic physical principles that underlie the universe. And yet they are the most versatile of radionuclide production systems, particularly for positron emitters.
In principle, positron emitting isotopes should be equally applicable to therapeutic applications, as beta minus emitting radionuclides. The availability of cyclotrons on site in many hospitals, provides for the convenient production of positron emitting radionuclides with a range of half-lives, some of which are far too short to be shipped conveniently.
This ability to use a range of half-lives in the therapy of human tumors with radiolabeled anti-tumor antibodies, is a considerable advantage because the biology of a particular targeting situation varies widely, and with it the optimal half-life that will lead to the best therapeutic index, in terms of the ratio of Rad dose to tumor and normal tissue, in terms of Rad dose.
Beta decay and positron decay (sometimes called beta-plus decay) is accompanied by the emission of a neutrino, which carries off angular momentum and some of the energy of the transition. In fact, there are a spectrum of energies for both particles, that accompany a single decay, and the amount of energy is usually described as the mean energy, which is about 1/3 of the maximum energy that the beta plus or minus particle may have. It is this energy of decay which determines how far the decay particle travels in tissue, and how much energy is deposited in the tissue.
For all practical purposes, beta minus and beta plus decay at the same energy of emitted particles, deliver very nearly identical amounts of radiation to the tissues. When the beta minus comes to rest in tissue, it is usually captured in the electron cloud of a nearby atom, but the positron combines with an electron, and annihilates, with the production of two gamma photons of 511 KeV, that comes off at 180 degrees from one another. Both lose energy in tissue principally by collision with orbital electrons, until they come nearly to rest.
In addition to the above art the following findings were also described in the past. Scheinberg and Strand disclosed a method for high resolution gamma ray imaging of mouse tumors obtained with leukemia cell-specific monoclonal antibodies labelled with bifunctional radioactive metal chelates (Scheinberg, D. A. and Strand, M., Science 215: 1511 (1982)). This reference describes the possibility that labeled tumor-specific antibodies such as those labeled with Iodine-131 may have tumoricidal effects in reference to Orderet al, Cancer Research 40(A)Part 2: 3001 (1980). J.A.M.A. 259 (14): 2126-2131 (1988) is a review article describing current and potential uses of positron emission tomography in clinical medicine and research related to oncology. The article surveys imaging procedures for evaluation of patients with malignant tumors and describes diagnostic tools for assessing the recurrence of malignant tumors after radiation therapy for the study of tumor biology (metabolic studies) and the like. However, no specific mention of positron emitting radiolabels attached to antibodies for the treatment of patients afflicted with tumors is mentioned.
Link et al described the production of Zr-89, a positron emitter, and its evaluation as a protein label (Link, J. M., et al., J. Labeled Comp. Radiopharm. 23(10-12):1297-1298 (1986)). Zr chloride or oxalate are reported to be transchelated non-specifically to plasma protein to which they bind weakly. In addition, the excretion pattern of Zr-DTPA injected into animals is discussed. However, no mention is made in this reference of any therapeutic use of positron emitting labels for therapeutic uses.
U.S. Pat. Nos. 4,331,647, 4,361,544, 4,444,744, 4,460,561 and 4,460,559 to Goldenberg all utilize radiolabels such as alpha-emitters, beta-emitters or positron-emitters in general for tumor radiotherapy. These patents are all somewhat related to one another and claim from broad methods of tumor radiotherapy encompassing the injection of radiolabeled antibody which is specific to a marker (U.S. Pat. No. 4,361,544) to more limited methods of tumor radiotherapy comprising the peritoneal injection of the radiolabeled antibody followed with radiation of thermoneutrons directed to the tumor location (U.S. Pat. No. 4,361,544), a method of tumor radiotherapy comprising the peritoneal injection step described above, locating a tumor with a photoscanning device and the irradiation of the tumor locus with thermoneutrons (U.S. Pat. No. 4,444,744), to a method of tumor radiotherapy comprising the described peritoneal injection, the further injection of an indifferent immunoglobulin carrying a different radiolabel, localizing the tumor by photoscanning the indifferent immunoglobulin label and finally irradiating the tumor with thermoneutrons. Some radiolabeled antibodies also are required to carry a Boron-10 isotope. The antibodies may be monoclonals, polyclonals or fragments thereof. However, no use of I-124 radiolabeled antibodies is mentioned.
U.S. Pat. No. 4,454,106 to Gansow et al describes therapeutic and diagnostic methods utilizing metal chelate-labeled monoclonal antibodies. The metals employed are alpha-emitters, beta-emitters or Auger electron-emitting isotopes. The diagnostic techniques utilize positron-emitting metals as well as fluorogenic or paramagnetic metals. More specifically, Gansow et al claim a method of treating cellular disorders comprising administering to a patient monoclonal antibodies labelled with a chelate of a radiometals such as Ga-68 and Co-55, but neither Iodine-124 nor other positron emitters.
U.S. Pat. No. 4,737,579 to Hellstrom et al discloses novel monoclonal antibodies to human non-small cell lung carcinomas (NSCLS) displaying a high degree of binding to tumor cells. These antibodies are utilized in diagnostic methods, particularly suitable for the determination of the presence of a malignant condition in a patient. These antibodies are radiolabeled with "a label capable of producing a detectable signal" (see, claim 3) and gamma-emitters are apparently intended because there is a reference to imaging by means of a gamma camera (see, column 7, line 57). No mention is made in this patent however to either therapeutic uses or to the utilization of positron emitters.
U.S. Pat. No. 4,735,210 to Goldenberg relates to a lymphographic organ imaging method which requires the subtraction of a negative image produced using a gross imaging agent from a positive image produced with a specific antibody imaging agent. This is the latest of the Goldenberg patents and it seems to encompass only radiocolloids with a radiolabel such as Tc-99, Au-198, Hg-197, In-111, Ru-97, Ga-67, I-131 or I-123 and the like. Other elements are also called for in claims 12 and 13 of the patent. No mention again is there to therapeutic uses.
U.S. Pat. No. 4,466,951 to Pittman describes a primary amine-containing therapeutic or tracer agent which is prepared by binding cellobiose to the tracer agent to render it non-metabolizable and then binding an antibody to it to provide the capability of introducing the therapeutic or tracer agent into a cell. The therapeutic or tracer agent may contain Boron-10 and also be labeled with a radioisotope, preferably chelated to the primary amine. Radioisotopes such as I-131, I-125 and I-115 are utilized but I-124 is not mentioned (see, column 8, line 67 and column 11).
U.S. Pat. No. 4,659,839 to Nicolotti et, al describes a coupling agent for joining a paramagnetic or radionuclide metal with an antibody fragment by means of a chemical linking group (see, claim 1). The metal ion can be I-125 as well as a variety of paramagnetic ions (column 7, lines 30-46). Positron-emitting radionuclides are mentioned (see, line 45 of column 7) but not in the context of therapeutic uses.
Thus, the need still exists for better detecting and therapeutic methods specifically targeting antigens such as those associated with tumors which afford a greater accuracy and highly successful results.